1. Field of the Invention
The present invention relates to a light-emitting device used for various light sources such as a backlight source, a display, illuminations, etc. or photo sensors, and more specifically to a light-emitting device with higher reliability and preferable optical characteristics.
2. Discussion of the Related Art
Recently, high-luminance, high-power semiconductor light-emitting elements, or small-scale and high sensitivity light-emitting devices have been developed and used in various fields. Such light-emitting devices have characteristics such as low power consumption, small-scale, lightweight, and therefore they are used as a light source for a light-printer head, a backlight source of liquid crystal display, light sources of various meters, various scanning sensors, etc.
FIG. 23 shows one example of a light-emitting device. A plastic package, which has a recessed portion, formed with integrally inserted lead terminals used for the light-emitting device. An LED chip is provided as a light-emitting element on the lead terminal. A bottom surface is exposed in the recess portion by die-bonding. Each of the terminals of the LED chip is electrically connected to the lead terminal by gold wires, etc. provided in the package. The LED chip provided in the recess portion is molded by a transparent member, which becomes rigid after curing. Consequently, the LED chip and the wires, etc. provided in the package can be protected from moisture, external forces, etc. in the environment. As a result, a highly reliable light-emitting device is achieved.
Since the light-emitting devices have become more widely used in various applications, they are now being used under more severe circumstances and conditions. For example, in applications for aircraft or automotive vehicles, the light-emitting devices are used even as high as +80 degrees Celsius or more, or as low as −20 degrees Celsius or less, depending on the outside air temperature. Further, they are subjected to outside air pressure, thermal shocks, as well as vibrations. In such cases, thermal stress makes each component repeatedly expand and contract. Therefore, their structural integrity deteriorates. Thus this arrangement has a negative influence on their optical characteristics and reduces their reliability. In addition, newly developed light-emitting elements, which can emit light in the near-ultraviolet range with high-luminosity, are now available. It is important for these lights to reduce deterioration of each element caused by light emission in the above ranges.
Recently, resin with a siloxane-coupling has received attention since it can prevent being cleaved by the light. The resin is impervious to the light in the above range and sufficient heat, and has a high flexibility.
While it has a high flexibility, its surface is soft, and thus its mechanical strength is low. Therefore, it is not suitable to use in an exterior member of a light-emitting device. Further, it has tackiness on its surface allowing foreign particles to be adhered thereon. Consequently, it is not suitable for a light-emitting surface.
Japanese Patent Laid-Open Publication Kokai No. 2000-150968 shows a light-emitting device that is composed of a package with a high thermal dissipation. A light-emitting element is provided on a metal base member and a rigid cover with a flexible light-resistant member is provided inside an opening wall, covering the light-emitting element. Such a light-emitting device having the above configuration is not only sufficiently impervious to light and heat, but also has sufficient mechanical strength against an external force.
However, when during the flexible member is covered by the rigid member as mentioned above, air bubbles tend to be mixed into the flexible member. Especially, if the flexible member is completely sealed by a rigid member such as metal, glass, or the like (which are not permeable to gas), the air bubbles reduce the thermal stability of the flexible member. Therefore, the flexible member cannot release thermal stresses and the rigid member in contact with the flexible member might be damaged. Furthermore, if the air bubbles were trapped at the interface between the flexible member and the rigid member, the air bubbles would peel these surfaces apart and then an air layer would be formed therein. This can cause a reduction in the light-emission power and a fluctuation in the optical characteristics of the device.
The present invention is devised to solve the above problems, and therefore, is aimed at providing a light-emitting device with a higher reliability and stable optical characteristics.